Will having 400,000 THB in a Bangkok Bank account affect my visa application from England as I'm married to a Thai national?

The bank book isn't a problem. I applied for my Non o married to a Thai whilst in UK and got it back in 2 weeks. I was asked for a letter from the bank who I guess guarantees you have it,as the print out from the bank wasn't what they want (even though the bank manager said he knew exactly what I needed😅) I think it was made easier the fact I'd already entered Thailand on a non o as I believe people have had issues changing from different visas, especially the over used 'Covid Extensions 😏Its easy enough to apply for the non o from uk imo. For uploading the ID'S of partner use a free collage app as when uploading multiple images on the evisa site it only gives you impression you can upload one. They accepted my collage of about 6 different images on 1 image file. Good luck

Did you check Bangkok Bank London branch with what they can do for you?

Or see if an online statement you can get is enough.

Or enter Thailand on visa-exempt or SETV and apply for in-country Non-O..

Why not arrive on a 30 day entry.... Apply for a 90 day non 0 after a week ..Run that down to 30 days left and apply for marriage visa. No big deal, the paper work easy as long as u have that seasoned 400 thousand in Thai bank account. Takes the insurance problems out . What u do with health insurance then up to u. If u want advice on health insurance contact amarak namak on Facebook, she will take care of any insurance needs. My personal opinion it is easier for u to do inside Thailand. Only my opinion.

You may also print your online bank statement if you do not have the book.

Getting a visa issued outside of Thailand is different then getting a one year extension of stay that's issued inside Thailand.

Given that you have 400K THB in a Thai Bank Account, I believe the Thai Consulate in the UK will have no problem issuing you a single entry Non-O visa based on marriage to a Thai national.